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Magnetospheric Multiscale Observations of Electron Scale

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Magnetospheric Multiscale Observations of Electron Scale PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Magnetospheric Multiscale Observations 10.1002/2017GL075711 of Electron Scale Magnetic Peak Key Points: S. T. Yao1,2 ,Q.Q.Shi1, R. L. Guo3, Z. H. Yao4,5, A. M. Tian1, A. W. Degeling1, W. J. Sun3, J. Liu2, • Electron scale magnetic peak is X. G. Wang6, Q. G. Zong7, H. Zhang8 ,Z.Y.Pu7, L. H. Wang7,S.Y.Fu7, C. J. Xiao9, C. T. Russell10 , observed with the scale size of ~7 ρe 11 2 1 1 1 7 7 (~0.06 ρi) in the magnetosheath B. L. Giles , Y. Y. Feng , T. Xiao , S. C. Bai , X. C. Shen , L. L. Zhao , and H. Liu • Electron vortex is found perpendicular to the field line and the current system 1Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, is consistent with the magnetic peak Shandong University, Weihai, China, 2State Key Laboratory of Space Weather, National Space Science Center, Chinese • The magnetic peak is determined as 3 fl Academy of Sciences, Beijing, China, Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, magnetic bottle shape rather than ux 4 rope by a new method Chinese Academy of Sciences, Beijing, China, Mullard Space Science Laboratory, University College London, Dorking, UK, 5Laboratoire de Physique Atmosphérique et Planétaire, STAR Institute, Université de Liège, Liège, Belgium, 6Department of Physics, Harbin Institute of Technology, Harbin, China, 7School of Earth and Space Sciences, Peking University, Beijing, Supporting Information: China, 8Physics Department and Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA, 9State Key • Supporting Information S1 Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing, China, 10Department of Earth, 11 Correspondence to: Planetary and Space Sciences, University of California, Los Angeles, CA, USA, NASA Goddard Space Flight Center, Q. Q. Shi and R. L. Guo, Greenbelt, MA, USA [email protected]; [email protected] Abstract The sudden enhancements of magnetic strength, named magnetic peaks (MPs), are often observed in the magnetosheath of magnetized planets. They are usually identified as flux ropes (FRs) or Citation: Yao, S. T., Shi, Q. Q., Guo, R. L., Yao, Z. H., magnetic mirror mode structures. Previous studies of MPs are mostly on the magnetohydrodynamics (MHD) Tian, A. M., Degeling, A. W., … Liu, H. scale. In this study, an electron scale MP is reported in the Earth magnetosheath. We present a typical case (2018). Magnetospheric Multiscale with a scale of ~7 electron gyroradii and a duration of ~0.18 s. A strong magnetic disturbance and associated observations of electron scale magnetic fi peak. Geophysical Research Letters, 45, electrical current are detected. Electron vortex is found perpendicular to the magnetic eld line and is 527–537. https://doi.org/10.1002/ self-consist with the peak. We use multipoint spacecraft techniques to determine the propagation velocity of 2017GL075711 the MP structure and find that the magnetic peak does propagate relative to the plasma (ion) flow. This is very different from the magnetic mirror mode that does not propagate relative to the plasma flow. Furthermore, Received 20 SEP 2017 fi “ ” “ ” Accepted 22 DEC 2017 we developed an ef cient method that can effectively distinguish magnetic bottle like and FRs like Accepted article online 29 DEC 2017 structures. The MP presented in this study is identified as magnetic bottle like type. The mechanism to Published online 24 JAN 2018 generate the electron scale magnetic bottle like structure is still unclear, suggesting that new theory needs to be developed to understand such small-scale phenomena. 1. Introduction Magnetic peaks (MPs), characterized by a sudden enhancement of magnetic strength, have been widely observed in the space plasma. The large amount of previous research indicates that the MP is a very common phenomenon in the terrestrial magnetosheath (e.g., Elphic, 1995; Lucek et al., 1999; Roux et al., 2015; Soucek et al., 2008). Three types of the MPs are often observed in the magnetosheath region: flux transfer events (FTEs), the flux ropes (e.g., Huang et al., 2016; Karimabadi et al., 2014), and mirror mode peaks. FTEs are structures with a bipolar signature in the magnetic field component normal to the magnetopause, and an enhancement in field strength. It was originally described as the result time variations in the rate of reconnection at the magnetopause by Russell and Elphic (1978) and was followed by an updated description that FTEs are a general observational feature of magnetic reconnection at the magnetopause (e.g., Fear et al., 2008, 2009; Kawano & Russell, 1997; Rijnbeek et al., 1984; Wang et al., 2005, 2006; Zhang et al., 2012). Several ©2017. The Authors. models were developed to interpret the generation of the FTEs (e.g., the spatially limited reconnection, This is an open access article under the Russell & Elphic, 1978; multiple X line reconnection, Lee & Fu, 1985; Raeder, 2006; and single X line reconnec- terms of the Creative Commons tion, Scholer, 1988; Southwood et al., 1988). Based on in situ measurements, the morphology of FTEs is often Attribution-NonCommercial-NoDerivs License, which permits use and distri- described as flux ropes (FRs) (e.g., Eastwood et al., 2012; Xiao, Pu, Huang, et al., 2004). In the magnetosheath, bution in any medium, provided the the field lines of the FTEs are connected to the Earth on one side (e.g., Elphic, 1995; Roux et al., 2015). In some original work is properly cited, the use is specific situations, the FRs can be regarded as the 3-D version of 2-D magnetic islands (e.g., Fu et al., 2015, non-commercial and no modifications or adaptations are made. 2016) and their spatial size is of the order of a few Earth radii (RE). YAO ET AL. 527 Geophysical Research Letters 10.1002/2017GL075711 The magnetic mirror mode, a well-known phenomenon in which the field strength is anticorrelated with the plasma density and embedded within the ambient plasma flow, has long been of interest in space physics. Mirror modes are generated by the mirror instability, which is dominantly excited in high ion temperature anisotropy and large plasma beta environments, such as the solar wind and planetary magnetosheath (e.g., Joy et al., 2006; Soucek et al., 2008; Tsurutani et al., 1984; Yao et al., 2013). It is usually characterized by a series of magnetic holes (dips and troughs) and peaks (humps) (e.g., Winterhalter et al., 1994; Xiao et al., 2010, 2011, 2014; Zhang et al., 2008, 2009). From the first theoretical studies of the instability (Tajiri, 1967; Vedenov & Sagdeev, 1958), the importance of mirror mode and its physical effects have been recognized. Previous studies mainly focus on large-scale mirror mode peaks and the FRs, namely, on the MHD scale. It is noteworthy that there is a counterpart of the mirror mode instability which is so-called “electron mirror instability.” It was developed more than three decades ago (Basu & Coppi, 1982, 1984) in a hot electron and cold ion plasma environment. Similar to the mirror mode, this instability is excited by electron anisotropy and described by fluid-like equations (e.g., Basu & Coppi, 1982, 1984; Pokhotelov et al., 2013). Therefore, the instability and its excited structures are still in the MHD scale. In recent Magnetospheric Multiscale (MMS) (Burch et al., 2015) studies, ion scale FRs were observed during the reconnection at the magnetopause (Eastwood et al., 2016). Nonideal ion behavior and filamentary currents were exhibited. Huang et al. (2016) identified this kind of ion scale structure in the turbulent magnetosheath as a magnetic island. Intense wave activities and electron beams were found near the structure. Due to the low resolution of available data, previous studies have seldom referred to small size magnetic peaks. If ever the word “small” is used, the minimum size is the ion scale. In this study, we report a new kind of electron scale magnetic peak and show new observations of an electron scale physical process. Burst mode MMS data are used, which have 128 Hz sampling for magnetic fields from the Fluxgate Magnetometer (FGM) instrument (Russell et al., 2016), and 150 ms and 30 ms resolution for the ion and electron measurements, respectively, from the Fast Plasma Investigation (FPI) instrument (Pollock et al., 2016). The structure is observed in a spatial scale of ~7 ρe (electron gyroradius) or ~0.06 ρi (ion gyroradius) and lasted ~0.18 s. The plasma features are detailed in section 2.1. Current densities and electron distributions are analyzed in section 2.2. Three multisatellite techniques are used to determine the morphology, dimensionality, and propagation velocity of the peaks in section 2.3. In section 2.4 we present a simple and reliable method to distinguish “magnetic bottle like” structures (such as magnetic mirror and mirror mode structures) and “FRs like” structures, since they are very similar in space observations. This is followed by a summary and discussion section. 2. Observations 2.1. Plasma Features Figures 1a–1e show an overview of magnetic field and plasma properties around 11:11:26 UT on 27 January 2017. Figures 1a–1e show the ion and electron energy spectra, three magnetic field components (in geo- centric solar ecliptic (GSE) coordinates), and magnetic strength, plasma density, and amplitude of bulk velo- city, recorded by MMS1. MMS spacecraft were located at about [9.9,À1.9, 2.1] RE, and the four spacecraft approximately formed a regular tetrahedron with an average spacecraft separation of ~6 km. MMS1 crossed the magnetopause at~11:11:10 UT (the first dashed line), and remained in magnetosheath until ~11:11:57 UT (the second dashed line). The MP was observed at ~11:11:26.0 UT (in the blue shadow region).
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